In the design of a zoom lens for video camera for people's livelihood, as trends of taking advantage of a downsized imaging element, there are a trend of aiming for downsizing with the same zoom ratio (variable power rate) and a trend of aiming for higher power in zoom ratio with a practical size.
As an example of techniques for realizing the higher-powered zoom lens in the latter trend, there is one described in Japanese Patent Application Publication No. 8-5913. This zoom lens is composed of five lens groups in an arrangement of positive, negative, positive, negative and positive refractive power in order from the object side, and at least a second lens group and a fourth lens group are moved for zooming (varying power) and focusing to thereby obtain a zoom ratio of about twenty times.
However, if by making use of further downsizing of the imaging element, the power rate is increasingly made high, for example, if in order to obtain a zoom ratio of 40 times, the technique of Japanese Patent Application Publication No. 8-5913 is applied as it is, the following problems have arisen. Namely, fluctuations in aberration by zooming, chromatic aberration and spherical aberration at a telephoto end, and the like cannot be corrected. Therefore, in the technique of Japanese Patent Application Publication No. 8-5913, while the practical size of a zoom lens is maintained, the realized power rate is limited to about 20 times.
Consequently, in a technique described in Japanese Patent Application Publication No. 2000-105336, in order to correct the fluctuations in aberration by zooming, the chromatic aberration and spherical aberration at the telephoto end, and the like, which have been problems in realizing the high power rate, aspherical lenses are introduced to a third lens group and a fifth lens group and a number of materials each having a large Abbe number and abnormal partial dispersibility are used to thereby realize an angle of view of not less than 85 degrees and a zoom ratio of 40 times at a wide-angle end.
However, in the technique shown in the above-described Japanese Patent Application Publication No. 2000-105336, three sheets of special low-dispersion glass each having a large Abbe number and abnormal partial dispersibility are used. Since this special low-dispersion glass has a soft quality and low latent flaw resistance as is well known, latent flaws are easily caused during ultrasonic cleaning in lens manufacturing. Furthermore, since a thermal expansion coefficient of the special low-dispersion glass is large, when it is heated inside a vacuum chamber in a vapor deposition process for lens coating, and immediately after the vapor deposition, air is caused to flow into the vacuum chamber to quench, cracks easily occur. Therefore, the glass needs to be left inside the vacuum chamber for long hours after the vapor deposition to cool slowly, so that a vapor deposition cycle takes long, which causes a problem with productivity and further disadvantageously affects costs.
Accordingly, the zoom lens shown in Japanese Patent Application Publication No. 2000-105336 wherein three lenses made of special low-dispersion glass are used cannot be mass-produced, and thus unsuitable for zoom lens for people's livelihood.
Consequently, in light of the above-described problems, it is an object of the present invention to provide a zoom lens which can cover from a super wide-angle area to a super telephoto area with angles of view of not less than 67 degrees at a wide-angle end and not more than 1.6 degrees at a telephoto end, whose various aberrations can be favorably corrected while having a zoom ratio of about 40 times, and which is excellent in mass productivity, and an imaging apparatus using the zoom lens.